1. Field of the Invention
The present invention relates in general to camera calibration and more particularly to a method and a system for self-calibrating a wide field-of-view camera (such as a catadioptric camera) using a sequence of omni-directional images of a scene obtained from the camera.
2. Related Art
Cameras having a wide field of view are important in several diverse applications. By way of example, wide field-of-view cameras are used in applications such as surveillance, video conferencing, multimedia-based applications (such as games), autonomous navigation, advertising (especially in the areas of real estate and tourism) and scene reconstruction. Wide field-of-view cameras are used to capture most or all of a region of interest (or scene) while using a minimum number of cameras.
One type of wide field-of-view camera is a catadioptric camera. In general, a catadioptric camera uses a combination of reflecting surfaces (such as mirrors) and lenses to achieve an omni-directional view of a scene. These reflecting surfaces, however, tend to cause image distortion. Image distortion is inherent in a catadioptric camera because light rays at a periphery of the field of view are severely curved to allow the use of a smaller, inexpensive image detectors (such as a charge-coupled device or CCD). Otherwise, prohibitively large and expensive image detectors would be needed to capture the entire field of view. The image distortions tend to pull points toward an optical center of the camera, thus making straight lines within a scene appear curved in an image of the scene.
If optical characteristics of a catadioptric camera system are known, then the parameters that map the curves in the image into straight lines can be applied. Unfortunately, however, these parameters are seldom known accurately and optical characteristics tend to vary between particular camera systems. This requires that a catadioptric camera be calibrated in order to determine calibration parameters that will characterize incident light rays and thus account for optical distortion.
Several types of calibration techniques are used to calibrate catadioptric cameras. In one calibration technique the camera photographs a calibration pattern and uses the pattern to calibrate the camera. One disadvantage, however, of this technique is that the calibration pattern is large and using the pattern is awkward and time-consuming. Another calibration technique involves a user selecting a number of points along straight lines of the scene (which are curved in the image). One disadvantage with this technique, however, is that the user must take the time to supply the calibration system with the data needed to correctly calibrate the camera. In reality, both of these calibration techniques may be difficult to implement in consumer-based products because it is unlikely a consumer would want to or could correctly perform such complicated and time-consuming calibration techniques. Moreover, if the camera calibration is performed incorrectly any results obtained from the camera will tend to be erroneous.
Accordingly, there exists a need for a calibration method and system for a catadioptric camera that is accurate and simple to use. This calibration method and system should be capable of self-calibrating using images of a scene obtained from camera, thereby eliminating the need for calibration patterns or user-supplied points. In addition, this self-calibration method and system would provide a wide variety of calibration parameters to enable accurate results from the catadioptric camera. Whatever the merits of the above-mentioned systems and methods, they do not achieve the benefits of the present invention.